One of the packaging methods that is in common practice today is vacuum shrink packaging, according to which an article of interest is wrapped with a heat-shrinkable thermoplastic film in the form of a bag and the package is vacuumised, heat sealed and cut off along the edge portions. The obtained package is heat shrunk to provide a tight appearance. Heat shrink packaging has gained increased popularity because it adds to the aesthetic appeal of the packaged articles, thereby enhancing their commercial value, and because it permits easy packaging of articles even if they have complex shapes.
On the other hand, packages overwrapped in heat-shrinking film have the disadvantage that the film sticks so closely to the underlying surfaces and are in such tension that they are difficult to tear when the package must be opened.
Various methods have so far been proposed with a view to overcoming this problem and facilitating the opening of packages that are made of heat shrunk films. These include: i) forming a line of perforations in a specified portion of a heat shrinkable film so that the package can be opened by tearing along the line; ii) making a U-shaped hole in a specified site on the film and attaching a paper seal over the hole to form a seal label that serves as an aid in opening the package; and iii) additionally attaching a so-called “opening tape”.
However, these conventional methods have their own limitations. In the first approach, if the perforations overlie the contents of the package the perforations will grow in size upon heat treatment of the heat shrinkable film so that contaminants can potentially get into the package through the holes, thereby reducing the commercial value of the contents. In some cases, the perforations may rupture during handling. The second approach needs a special apparatus for making the U-shaped hole and, furthermore, the use of paper seals adds to the production cost. In any event, these two methods are not suited to vacuum packages, since an opening in a portion of the bag overlying the packaged item or items will lead to leakage of the vacuum. The third approach has the inherent disadvantage of being cumbersome.
An alternative arrangement for opening a shrink wrap bag is described in U.S. Pat. No. 3,516,537. In this proposal, an end seal of the bag is formed at a position remote from the bag edge to provide a tab and a tear notch is formed in the tab. A problem with this arrangement is that the tab shrinks at the same rate as the remainder of the bag when the film is shrunk around the contents, for example by at least 50%, and may therefore require a large portion of the pre-shrunk film material or be too small to readily grasp after shrinking
Another easy opening system for heat shrunk bags is disclosed in EP-A-0745539 where a heat shrinkable film wrap for a food product has an edge zone isolated from the remainder by a seal between opposed film layers, and a cut extending inwardly in the zone from one edge to form a tongue when heat-shrunk to facilitate opening. The film is wrapped around the product, heat sealed peripherally and then heat-shrunk. The zone is preferably separate from and adjacent to the package closure heat seal, and may be heat-sealed peripherally or over its entire area. The film may be in the form of a bag for packaging the product under vacuum, or allow packaging under a modified atmosphere.
This system has the disadvantage that, when shrunk, the protruding tongue may not be visible or may have an unpleasant appearance and may be difficult to grip.
An improved system for opening heat shrinkable, gas-barrier thermoplastic film is described in the related U.S. Pat. Nos. 6,260,705 and 6,499,598. In this proposal, a large tear tab is formed between a closed end of a bag and a first seal extending across the bag at a location remote from the closed end. A second, shaped seal is formed from the closed end of the bag and a pre-cut is provided in the film portion between the closed end of the bag and the second seal, midway along the closed end, to facilitate tearing. The portion of the tear tab between the first seal, the closed end and the second seal is airtight. The airtight portion may contain a gas such as air such that it balloons upon heat treatment, which is said to create two pillow shaped hems having a better packaging appearance. It is also suggested that the gas present in the airtight area may reduce the shrinkage of the thermoplastic film in that area.
A problem with the proposals in these U.S. patents is the substantial volume of film material that is taken up by the tear tab, and the associated cost. The large tear tab may also be considered to detract from the appearance of the heat-shrunk package and needs to be gripped by two hands to tear it.